Changing newsletter frequency

This really isn't a question of compliance or legality so much as a question of content.

Marketo itself doesn't know you're sending a "newsletter", but if you have enough content for different frequencies, say sorted by importance, you can choose someone's track based on program status or other characteristics (as long as higher- and lower-frequency tracks alike are below the global limit).

One approach that might help your team address this issue in the long run would be to build out a subscription management center. That way your recipients can elect what types of communications they want to receive, and on the subscription landing page you can add a note about how frequently you'll be sending out these communications. Once that's done, you can add a section to your newsletter that promotes the new subscription page and then send it out during your next regular cadence.

You need to have either of the following in order to lawfully email someone under GDPR:

1.) Explicit consent

2.) There needs to be a 'Legitimate Interest'

Here are CASLs requirements for commercial electronic messages: 

1.) Obtain consent (express or implied)

2.) Provide identification information

3.) Include an unsubscribe mechanism in each message.

There is nothing in CASL (or GDPR i believe) that requires you to state the frequency of messages, which has a lot more to do with exercising common sense and good business practices. If you already stated the frequency then try and stick to it. (Don't send more often than you say you will unless you're already communicating with the recipient about the change in frequency.) Otherwise you can expect your unsubscribes to go up.